Zimlets only work in relation to local folders
I have created a bug for this here: Bug 44577; Zimlets work only in relation to 'Local folders' (Zimbra Desktop 2)
The actions of Zimlets only apply to local folders, i.e. Birthday Calender reminder only creates a calender in local calenders, and only uses birthdays from local addressbook. This is particularly unhelpful. The user should be able to choose, or at least set the default account themselves, or else have an accordion-style display of zimlets at the bottom of each account tree. Vote if you want this changed quickly!

Zimlets are not deployed
In spite of following instructions in this thread: http://www.zimbra.com/forums/general...tml#post171216 , zimlets were not deployed, I only managed to deploy them by creating a '_dev' folder in '~/.zdesktop/zimlets-deployed' and copying the unzipped folders into it. Of course, this is only intended for development purposes, and not for general usage.
It is hoped that by release of Zimbra Desktop 2 final (GA) this bug will be implemented and allow users to deploy zimlets via the UI (vote to encourage it!): Bug 31419; Deploy Zimlets in Desktop (UI)

Spell correction in html emails not functioning
When composing an html email, right-clicking a work marked as misspelled does not offer the spell-check menu, it offers only the formatting menu. (If opening a plain text email this works ok).

Spellcheck not functioning
There seems to be no way of doing a mandatory spellcheck.

Mandatory Spellcheck blocks sending of emails
If a user sets spellcheck to be mandatory (in preferences, mail composition) then they cannot send emails as the spellcheck will not compete.
I have created a new bug here: Bug 45085 When mandatory spell check is enabled outgoing mail cannot be sent.

Default setting for responding to read-receipt requests is to refuse
The default settings is fairly well hidden in preferences, it should be better exposed, and it should be set as default to ask the user whether to accept or refuse, not just silently refuse, or most users will never know any better. The first time a user is asked for a read-receipt they could be requested to update the setting for future.

Lack of access to internationalised dictionaries or instructionsThis would be addressed by this enhancement bug: Bug 35874: provide same way of installing/switching dictionaries as FF does - vote if you want it!
In the meantime, I have managed to install an internationalised dictionary (in my case for the en-GB), however I had to resort to using instructions and dictionaries which were linked from a Zimbra wiki page which is no longer available (I used the Google cache of the page). There needs to be a widget in preferences where the user can automatically download and install the necessary dictionary and update settings, from within preferences. There should be documentation at least in the wiki also.

Performance is greatly enhanced in ZD2, however the following suggestion might benefit some power users:
Lazy loading user-customisation
Vote if you want it! - I have created an enhancement bug here: Bug 44576; Lazy loading - user customisation
The user should have ability to turn-off lazy loading for those modules considered necessary - i.e. Contacts (Address book) if they are in constant usage. This facility would be in preferences with a warning about potential impact on overall performance.

May want to check out this post:http://www.zimbra.com/forums/error-r...up-screen.html
This is a recent issue (late 2009) with zimbra desktop 1, that seem sto also be an issue with zimbra desktop 2. It seems to be some kind of interaction with prism, and possibly an itunes update, but it is unclear that if this is the case with zimbra desktop 2 - The above post has references to other posts which seem to me to describe similar issues in zimbra desktop 1 and some workarounds

Mandatory Spellcheck not functioning

I've created a new bug report: Bug 45085 When mandatory spell check is enabled outgoing mail cannot be sent.

Until spellcheck is working, users should not enable mandatory spellcheck
(preferences>mail>composing>'mandatory spellcheck before sending a message'

When this is enabled, users cannot send messages. If this occurs, close the message (by clicking the top right corner of the compose tab, as you will not be able to click 'cancel'). Zdesktop will offer to save the message as a draft. Go to preferences and disable the feature, until this bug is fixed. You should be able to send mail again. Resend any unsent messages from drafts.